FIG. 1 shows a conventional circuit 100 for measuring a cell voltage of a cell in a battery. The circuit 100 includes a battery 110, a level shifter 120, a resistor group 130, a capacitor group 180, a measurement unit 140, a current generator 160, and a switch group 170. In the example of FIG. 1, the battery 110 includes cells 111-113 coupled in series. The resistor group 130 includes resistors 131-134. The capacitor group 180 includes capacitors 182, 184 and 186. The resistor group 130 and the capacitor group 180 filter noises of signals transmitted between the battery 110 and the switch group 170. The switch group 170 selects a cell from the cells 111-113. The level shifter 120 provides a shifted voltage proportional to the cell voltage of the selected cell. The measurement unit 140 calculates the cell voltage of the selected cell according to the shifted voltage generated by the level shifter 120.
The level shifter 120 consumes currents from the battery 110. By way of example, when the cell 111 is selected, the level shifter 120 consumes a current I′cons1 flowing from the positive terminal of the cell 111, through the resistor 131 and the switch group 170, and to the level shifter 120. The level shifter 120 also consumes a current I′cons2 flowing from the negative terminal of the cell 111, through the resistor 132 and the switch group 170, and to the level shifter 120. The voltage drops across the resistors 131 and 132 adversely affect the accuracy of the cell voltage measurement. To reduce such impact, the current generator 160 generates a compensation current I′comp2 flowing through the level shifter 120, the switch group 170 and the resistor 132 and to the negative terminal of the cell 111. Thus, if the compensation current I′comp2 is substantially equal to the current I′cons2, the voltage drop across the resistor 132 can be ignored.
However, a difference between the positive terminal voltage of the cell 111 and the supply voltage of the current generator 160 may not be large enough such that the current generator 160 may not be able to generate a current flowing into the positive terminal of the cell 111 to compensate the voltage drop across the resistor 131. Thus, the voltage drop across the resistor 131 still has an adverse effect on the accuracy of the cell voltage measurement.